20. Carbohydrates

Draw a Haworth projection for α-D-altrose.

D-ribose is an aldopentose sugar that is found in the DNA. It commonly exists as a five-membered β anomer. Draw D‑ribose in its cyclic hemiacetal form.

In the monosaccharide hemiacetal shown below number all the carbon atoms, identify the anomeric carbon atom, and identify it as the α or β anomer.

The cyclic structure of D-idose, an aldohexose, is shown in the margin. Convert this to the straight-chain Fischer projection structure.

Identify the following as diastereomers, enantiomers, and/or anomers.

(a) β-D-fructose and β-D-fructose

(b) D-galactose and L-galactose

(c) L-allose and D-glucose (both aldohexoses)

D-Talose, a constituent of certain antibiotics, has the open-chain structure shown next. Draw d-talose in its cyclic hemiacetal form.

L-Fucose is one of the naturally occurring L monosaccharides. It is present in the short chains of monosaccharides by which blood groups are classified. Compare the structure of L-fucose shown in the margin with the structures of α- and β-D-galactose and answer the following questions.

d. "Fucose” is a common name. Is 6-deoxy-L-galactose a correct name for fucose? Why or why not?

Draw the structure of the α and β anomers that result from the reaction of methanol and ribose. Are these compounds acetals or hemiacetals?

In its open-chain form, D-mannose, an aldohexose found in orange peels, has the structure shown here. Coil mannose around and draw it in the cyclic hemiacetal ⍺ and β forms.

D-Fructose can form a six-membered cyclic hemiacetal as well as the more prevalent five-membered cyclic form. Draw the α isomer of D-fructose in the six-membered ring.

What are the kind and number of atoms in the ring portion of the Haworth structure of glucose?

Draw the Haworth structures for α- and ß-D-glucose.

Identify each of the following as the α or ß isomer:

a.

The sugar d-gulose is a sweet-tasting syrup.

b. Draw the Haworth structures for α− and β-D-gulose.

Identify the following carbohydrates as the ⍺ or β anomer:

(b)